This invention relates generally to low pressure hydraulic cylinders and particularly to hydraulic cylinders of the pneumatic type that are directionally controllable by the application of a compressed medium, such as air, to a piston. While the invention will be described in conjunction with an air cylinder, it will be appreciated that it is equally applicable to any type of low pressure hydraulic cylinder having suitable well known modifications to provide a closed system for the hydraulic fluid.
Pneumatic cylinders are well known in the art for mechanically positioning levers and gears in response to control signals or other inputs and for moving objects. They are extensively used in industrial machinery and packaging equipment. The air cylinder generally comprises a cylindrical tube or body, within which a piston is movable. The body has end portions or caps that close off the cylinder and have one or more air fittings for admitting compressed air to move the piston within the body. A threaded bolt for mounting the cylinder to a base or other suitable mounting device is generally affixed to one end cap. The piston has seals for maintaining pneumatic pressure within the body during operation. The other end cap may contain a suitable bearing for supporting a piston rod that is attached to the piston, for smooth movement. In a single acting cylinder, the piston is biased toward one end by an internal spring. In a double acting cylinder, the piston is freely movable back and forth and its direction is controlled by applying compressed air to different sides of the piston. Such cylinders have two air inlet fittings.
An air cylinder arrangement must include some means for controlling air flow to the body. This is generally accomplished with an air valve that, in turn, is operated by some type of control signal on control input. Control of an air cylinder may be accomplished mechanically, electrically, or pneumatically. In a simple mechanical arrangement, the air valve may be activated by a cam or triple lever on a moving object to direct or control compressed air flow. In an electrical control system, a solenoid actuates the valve to control the air flow in response to a switch or relay closure. In a pneumatic control system, a pilot cylinder is supplied with low pressure air to actuate the valve mechanism which, in turn, controls air flow to the cylinder.
The air cylinder bodies are generally fabricated of relatively thin material. If the body is of stainless steel, the body wall may be extremely thin and prone to damage from accidental contact with other objects. Any disruption of the body wall negatively affects cylinder operation because the piston needs a smooth interior cylindrical surface to function. It is not unusual for a manufacturer to provide an outer protective sleeve for air cylinders intended for use in a hostile environment, i.e., where the likelihood of physical damage is high. The protective sleeve for the cylinder body is usually made of thick aluminum to provide physical protection for the relatively thin wall air cylinder.
Use of an air cylinder in such an environment requires not only an air cylinder and control valve assembly, but a protective sleeve for the cylinder body. It should be apparent that the overall cost is increased significantly, especially since the valve is often more expensive than the cylinder and a protective sleeve adds even more cost. Furthermore, a typical installation of an air cylinder and an air control valve involves many fittings, connections and plastic piping and generally takes up significant space. There is clearly a need in the art for a simple, low cost, space efficient, controllable air cylinder mechanism.